Referring to FIG. 1, a number of conventional deep draft semi-submersible floating production platforms supporting steel catenary risers (SCRs), and using spread mooring for station keeping have previously been successfully designed and installed. The conventional semi-submersible has proven to meet the motion suppression requirements for SCRs efficiently, and has encouraged the possibility of developing technology for facilitating top-tensioned risers (TTRs) for field development.
Top-tensioned risers with dry trees have conventionally been supported by either a tension leg platform (TLP) or a “Spar” platform. Riser stroke induced by vertical motion of the host platform is compensated through tensioners by means of hydro-pneumatic hydraulic systems designed to control riser tension within allowable values.
TTRs and dry trees are field-proven technology for both TLP and Spar, but have not previously been utilized on a semi-submersible production unit. There is an increasing trend for drilling with high-pressure (HP) risers and surface blow-out preventers (BOPs) from semi-submersible platform units. This trend represents a similar but more challenging function than a conventional dry tree system with TTRs.
In addition to the conventional semi-submersible platform, a few configurations have previously been described. For example, U.S. Patent Application Publication No. US 2003/0095839 A1 describes a floating platform for offshore drilling and production with octagonal pontoon, with specific attention to a preferred hull draft and the ratio of draft to column center-to-center distance. In U.S. Pat. No. 7,467,912 B2, an extendable draft platform (EDP) is characterized in that each of the columns comprises an upper portion having a first diameter, and a lower portion having a second diameter in order to move vertically in the column well. As another example, U.S. Patent Application Publication No. US 2009/0114139 A1 describes a dual-column semi-submersible platform having vertical columns arranged in pairs, with one of the pair disposed a distance outward from the other.
However, the need remains for a semi-submersible production and drilling platform unit using TTRs with dry trees or direct vertical access to wells, and addressing the total value and efficiency of the integrated system during entire life cycle of the product, including design, construction, integration and installation, operation, and decommission, as well as component reliability and safety.